As described in Patent Document 1, conventional press-less processes consist of the following procedures: filling a mold with an alloy powder having an average particle size of 2 through 5 μm in such a manner that the filling density becomes 2.7 through 3.5 g/cm3; placing a lid on the mold; applying a magnetic field to the powder for orientation; sintering the powder; and taking out the sintered compact from the mold to perform an aging treatment. Although the method of measuring the aforementioned average particle size is not explicitly stated in Patent Document 1, it was probably measured using Fisher's method which was commonly used at the time when the document was filed.
Conventionally, materials used for the mold include Mo, W, Ta, Pt, and Cr, which are considered to be preferable examples of metals that do not react with an alloy powder. However, the inventor of the present invention has noticed the significant problem that all of these metals have one or more of the following three disadvantages: (i) they are expensive, (ii) they are difficult to be machined, and (iii) they will be embrittled once heated.
Given this factor, the inventor of the present invention has devised the use of Fe—Ni alloy such as stainless steel or Permalloy, which are not mentioned in Patent Document 1, as the material of the mold (Patent Document 2).
It had been known that, in mass-producing a NdFeB sintered magnet, if a compact made by pressing an alloy powder is put on a metal plate or in a metallic container and is sintered, the alloy powder reacts with or strongly adheres to the Fe—Ni alloy and the magnet after the sintering is considerably deformed. This is probably the reason why a Fe—Ni alloy was not mentioned as a material for the mold in Patent Document 1. The inventor of the present invention has solved the problem regarding the reactivity with an alloy powder by coating the inside of a mold, and thereby they have devised a mold using a Fe—Ni alloy which is inexpensive, easy to be machined, and will not be embrittled (Patent Document 3).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H07-153612
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2007-180375
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2007-180373